Out of curiosity (and you may have gone through this already) - how did you apply the 10.5.1 update? Have you tried reapplying the combo update? This problem isn't universal - probably the majority of people have NOT had this issue, so it's hard to say whether it's something in the 10.5.1 update or some other factor that may not appear to be related, but in fact is.
What happens if you start up as a different user? Or in Safe Mode? Any differences? Does Disk Utility see the drives? Can you unmount them and then mount them again? (My externals disappear and then reappear in the sidebar when I do this, as one would expect)
Have you tried trashing the Finder .plist and then restarting? Resetting the PRAM (it does store info about the startup volume, so might be worth a shot)
These may all be long shots, but possibly worth a go if you haven't tried them yet.
Actually, I haven't really done any troubleshooting with the problem. Partly because I've been busy. Partly because after upgrading to Leopard I've had a few quirky issues like this. (I just upgraded the system, not archive and install. Plus, I am always messing with my system, installing/uninstalling software and stuff. So it's probably me, not necessarily Leopard.) And partly because this issue hasn't really disrupted my work flow enough.
I was looking for a quick fix and thought I found it with that AppleScript.
To answer your questions - The issue is for all the user accounts. Haven't tried safe mode. Disk Utility can see/verify/repair/mount/unmount the drives. Haven't reset the PRAM and haven't trashed the finder.plist file. So now I will need take some time and troubleshoot to figure it out.
